1. Technical Field
The present disclosure relates to electric power steering apparatuses mounted on vehicles, such as automobiles, and particularly to an electric power steering apparatus with improved steering feels at an early stage of steering.
2. Related Art
The electric power steering apparatus detects torque of steering operation by a vehicle operator, and generates assist torque by an electric motor according to the detected torque. It is known that such electric power steering apparatus is provided with a torque amplifying mechanism that utilizes a set of a worm and a worm wheel (i.e., worm drive) in preparation for a case where a large assist torque is required, for example, when the operator performs a stationary steering (a steering operation when the vehicle is not traveling) or a sudden steering.
Such electric power steering apparatus may cause a point of inflection in the steering torque before and after the assist torque is generated by the electric motor, resulting in a nonlinear steering force and a deteriorated steering feel. That is, the operator senses a resistance when the steering torque passes through the point of inflection at the early stage of steering, and then senses a feel that the steering force is reduced in a stepped manner. Therefore, the operator finds the vehicle with a low quality of steering feels. Such a change in the steering torque may be caused by a reduction in a coefficient of friction at the engagement sections (sliding parts) of the worm drive when the assist torque is generated by the motor because the engagement sections transits from a static friction state to a dynamic friction state.
For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2004-299492 discloses a conventional technology of the electric power steering apparatus for improving a steering feel at an early stage of steering. The electric power steering apparatus changes a steering force characteristics model for setting a target steering force between an early stage of steering transitional phase and other steering phases in order to obtain an ideal steering feel both at the early stage of steering transitional phase and other steering phases.